Composite coated steel structure for corrosion resistance

ABSTRACT

A composite coated steel structure having a carbon steel or stainless steel base and cladding of tantalum or columbium to provide corrosion resistant surfaces for many uses. The composite includes a containment substrate coating interposed between the base and the cladding to prevent diffusion of the cladding material into the base. The containment coating has greater stability with respect to the base than the cladding material to prevent adverse reaction under high heat conditions.

United States Patent Glaski 1 June 17,1975

[ COMPOSITE COATED STEEL STRUCTURE FOR CORROSION RESISTANCE [75] Inventor: Frederick A. Glaski, Torrance,

21 Appl. No.: 366,284

Related US. Application Data [60] Division of Ser. No. 284,639, Aug. 29, 1972, Pat. No. 3,784,403, which is a continuation-impart of Ser. No. 177,929, Sept. 7, 1971, Pat. No. 3,767,456.

3,219,474 11/1965 Priceman et a1. 117/71 M 3,628,924 12/1971 Nishio et al 29/196 3,767,456 10/1973 Glaski 117/71 M 3,784,403 l/1974 Glaski H 117/71 M OTHER PUBLICATIONS Sachs, (3., et al.; Practical Metallurgy; Cleveland (ASM) 1951 p. 294, [TN 665 S 240].

Barrett, C., et a1.; Structure of Metals; New York, 1966 p. 536, [TN690 B3].

Primary Examiner-Walter R. Satterfield Attorney, Agent, or Firm-Barnes, Kisselle, Raisch & Choate [57] ABSTRACT A composite coated steel structure having a carbon steel or stainless steel base and cladding of tantalum or columbium to provide corrosion resistant surfaces for many uses. The composite includes a containment substrate coating interposed between the base and the cladding to prevent diffusion of the cladding material into the base. The containment coating has greater stability with respect to the base than the cladding material to prevent adverse reaction under high heat conditions.

1 Claim, 1 Drawing Figure COMPOSITE COATED STEEL STRUCTURE FOR CORROSION RESISTANCE This application is a division of my copending appli' cation entitled Process for Cladding Steel. Scr. No. 284,639. filed Aug. 29. I972. novi' LES. Pat. No. 3,784,403. issued .Ian. 8. I974 which in turn was a continuation'in-part of my copending application now entitled Chemical Vapor Deposition of Steel with Tantalum and Colunibium." Scr. No. 177.929. filed Sept. 7, l)7l.and now US. Pat. No. 3.767.456. dated Oct. 23. 1973.

This invention relates to a Composite Coated Steel Structure for Corrosion Resistance.

There are certain applications of steel in which the product would be much enhanced by a cladding of tantalum on the surface to increase resistance to corrw sion. The problem is to find an acceptable and reproducible bond between the tantalum and the steel and. secondly. to achieve uniformity over all the surfaces of a complex shape or a hatch of complewshaped hardware.

An attempt to deposit tantalum directly on steel containing carbon results in a poor bonding inasmuch as the tantalum combines with the carbon of the steel at an excessive rate leaving Kirkendall type porosity in the steel. It is possible to provide an excellent bond between tantalum and pure metals such as molybdenum. chromium and iron; but as the carbon content of the steel increases, the bond becomes less satisfactory. In steels such as the stainless variety the carbon content may be reduced to the point of not causing objectionable combining with tantalum but the high nickel con tent creates the problem here. In a CVD process. the nickel will diffuse rapidly into tantalum leaving again the objectionable porosity.

The present invention contemplates a coating on steel which will protect the surface against the deterio ration when tantalum is deposited by chemical vapor deposition, It is, therefore, an object of the invention to provide a process for cladding steel with tantalum or possibly columbium which is consistently successful and uniform.

It is a further object to provide a product of a tantalum clad steel resulting from the process and to provide a process which insures uniform results on complicated shapes.

Other objects and features of the invention relating to details of the process and the product will be apparent in the following description and claims in which the principles of the invention are set forth in connection with the best mode presently contemplated for the practice of the invention.

A DRAWING accompanies the disclosure and a single view thereof may be described as a diagrammatic illustration of an apparatus for accomplishing the present invention.

In connection with the problem of tantalum cladding of carbon containing steel, we propose to establish a containment surface on the steel which will prevent carbon diffusion from the steel substrate to the tantalum being deposited thereon.

By way of definition, an overcoat on a particular sub-strate is defined as the deposition of a measurable thickness of material on a surface. All electroplated coatings are overcoats; and. similarly. hydrogenreduced refractory metal halides in a chemical vapor deposition process results in an overcoat. In these de positions. all the components of the coating are provided by the plating bath or the plating gas and none are provided by the substrate.

We propose a containment surface on a substrate which might be defined as a displacement diffusion coating in which at least one of the constituents of the coating is provided by the substrate. In this process. the coating does not build up on the surface to any appreciable extent but rather grows into the surface. For example. when titanium is used. this may be referred to as a "titanizing process." The -izing" or -ized" suffix placed on the end of the name of the element deposited from the gas denotes such a reaction.

In connection with titanizing. there is a reaction of the titanium with the carbon in an atmosphere of TiCl in the temperature range of 90(]l l()0(. which forms a thin diffusion barrier of titanium carbide. The TiCl reacts with the surface carbon on the carbon steel substrate to form TiC and once all surface carbon is reacted, the titanium deposition ceases. Thus. there is a definite limitation on the thickness of the displacement diffusion coating. Since titanium carbide is thermodynamically more stable than TaC. the pure tantalum may be deposited without further carbon diffusion from the sub-strate In the preparation of the displacement diffusion coating. there may be a reaction with free carbon on the surface with the TIC], but, in general, the carbon is available primarily from metal carbides in the sub-strate which are less stable than TiC. Nearly all the metal carbides normally occurring in carbon steel and low alloy carbon steel are less stable than TaC and thus displacement diffusion coating is required.

While titanium has been mentioned above. it is possible to use a containment coating of a metal selected from Groups IVb and Vlb. iron, and cobalt. of this group, for example, titanium carbide and ZrC are both more stable than tantalum carbide. After this containment coating is applied. it is possible to overcoat with pure tantalum in a CVD process without further carbon diffusion from the sub-strate.

In connection with stainless steels which are high in chromium and nickel. the problem with carbon diffu sion is reduced since the carbide with chromium. namely, Cr,-C is very stable. On the other hand. the nickel content of the stainless stell will diffuse uncontrollably into a tantalum clad coat. Thus, for these steels, an overcoat of the pure metal to which tantalum may be bonded in a CVD process must be used. In some instances, a plating process may be used and in other instances a plasma spray can provide a coating to which tantalum may successfully bond. In each case, care must be used to obtain a good bond between the sub-strate and the overcoat. With electroplating. careful pre-plating procedures must be observed in the cleaning of the part and well controlled plating steps must be utilized. In the plasma spraying, a coating of pure iron. chromium. cobalt, molybdenum or tungsten may be applied; in fact. any of the metals from Group IVb and Group Vlb as well as iron or cobalt can be practically applied in this way.

Once the pure metal overcoat is applied to the stainless stecl, the nickel diffusion is blocked and the CVD coating or clad of tantalum can be readily applied using conventional procedures.

It will be appreciated that carbon steel can also be plated or plasma sprayed with pure metals as above described to block carbon diffusion. With steel castings. for example, they are sometimes ground out and plasma coated with iron to remove the surface porosity and then titanized to stabilize any carbon that may have diffused into the plasma coat. Then the tantalum clad may proceed with a resulting good quality and uniform Coat.

With respect to the use of plasma spray of iron or stainless steel. the carbon diffusion is not as critical a problem since the iron intcrlayer remains pure iron and the bonding of the CVD tantalum readily occurs with out titanizing.

It has also been noted in tantalum cladding that deposition has occurred preferentially on certain areas of exposed parts rather than uniformly or in the areas where it is most generally desired. Tantalum cladding has been much improved by a control of the mixing of the tantalum CI and the hydrogen reactant gases.

In the chemical vapor deposition process which otherwise proceeds in accordance with accepted procedures in a reduction reaction, better results have been obtained when the TaCL- is introduced to the chamber in the normal manner as indicated in the drawing. However. the hydrogen is introduced through a separate feed line or a plurality of feed lines which terminate immediately upstream of the object to be coated. In other words, the gases are intentionally separated until they impinge on the object to be coated. This provides excellent control in putting the tantalum deposit where it is desired. Utilizing this technique. uniform tantalum claddings have been obtained on batches of cast valve bodies. for example, during a 45-minute deposition pe' riod in a system that formerly could not generate tantalum coverage over all areas of the valve bodies during a 5hour period of deposition. Thus, there is a great savings in materials and expense in performing the process as well as a much more desirable and uniform result. In some instances, a control of this combined flow which is brought together immediately upstream of the area to be plated can be improved by using multiple exhaust ports which are balanced to direct the flow to certain areas of the parts to be exposed.

While the above description has referred to tantalum cladding, it will be appreciated that columbium (niobium) is chemically similar and may be applied as a cladding coat in the same way as has been described in connection with tantalum. The same problems which require a containment coat for tantalum exist in relation to columbium. There is a considerable savings in the use of columbium since it is a less expensive metal but, on the other hand, tantalum has a broader applicability as a corrosion resistant material.

The term "coating or coat as used in the specifcation and claims has reference to the application, by CVD. plating, or plasma spray. to a sub-strate surface of a metal in an extremely thin layer under conditions as above described in which the metal unites with the surface carbon of the sub-strate to form a carbide having greater stability than tantalum carbide. The surface is thus prepared for the application of tantalum by chemical vapor deposition to the extent that there will be no perceptible formation of tantalum carbide at the bonding face. The above description of coating relates to metals from Group IVb and the coating may also include metals from Group Vlb, iron and cobalt applied as a pure metal to reduce the carbon diffusion during the tantalum coating.

Following are three examples of a process used for steel cladding with sub-strates of differing carbon and stainless steels and various treatments prior to the titanizing and the application of the tantalum clad.

EXAMPLE I Cladding of 6 inches l.D. X 24 inches long pipe spool made of type A-l06 Carbon Steel 1. Sand blast surface to remove scale and roughen surface for subsequent plasma sprayed coating.

2. Plasma spray 0.002 inch thick coating of iron on ID. and flange seal faces.

3. Place in CVD (chemical vapor deposition) furnace. evacuate with vacuum pump and heat to titanizing temperature of 1,070C. in argon.

4. Bubble hydrogen at 2 liters/minute through liquid TiCl and mix with the resultant gas 7 liters/minute of pure hydrogen (STP) and flow the mixture through spool, over flange seal faces and 1D. surface for onehalf hour at l,070C. and -23 inches Hg. pressure.

5. After titanizing, adjust temperature to l.0l0 1.040C. range and begin tantalum deposition by flowing 3 liters/minute of chlorine through heated tantalum chips to form TaCl and mixing the resultant gas with 18 liters/minute of hydrogen, passing it through the spool at 34 torr pressure. All gas flows at STP.

6. After one-half hour. adjust gas flows to 7.5 liters/- minute of chlorine and 45 liters/minute of hydrogen. both at STP.

7. Total tantalum deposition time 6 V2 hours.

8. Turn off chlorine and hydrogen and cool in argon.

9. Resultant deposit thickness from 0.009 to 0.015 inch over both flange seal faces and ID. of spool.

EXAMPLE 2 Cladding oftype 31658 pump housing with l inch ports and flanges and 4 inches diameter cavity 1. Degrease and deseale in HCl base commercial descaler solution.

2. Place in CVD furnace, evacuate with vacuum pump and heat to titanizing temperature at l,000C. in argon.

3. Bubble hydrogen at 2 liters/minute (STP) through liquid TiCl and mix the resultant gas with 7 liters/mi nute of pure hydrogen and flow the mixture through pump housing; over flange seal faces and inside surface for 1 hour at l,000C. and -23 inches Hg. pressure.

4. After titanizing, adjust temperature to l,Ol0 1.030C. range and begin tantalum deposition by flowing l.5 liters/minute of chlorine through heated tantalum chips to form TaCl and mixing with 9 liters/minute of hydrogen, passing the mixture through the pump housing at 30 torr pressure. All gas flows at STP.

5. Total tantalum deposition time 5 & hours.

6. Turn off chlorine and hydrogen and cool in argon.

7. Resultant deposit thickness from 0.012 inch to 0.020 inch over both flange faces and inside surface of pump housing.

EXAMPLE 3 Cladding of type I2 I 5 carbon steel thermowell, three-fourths inch diameter X 4 inches long well with 2 /2 inches diameter (flat-toflat) X 2 inches long hexagonal base. Total length 6 inches 1. Degrease and descale in HCl base commercial descalcr solution.

2. Place in CVD chamber. evacuate with vacuum pump to heat directly by induction to titanizing temper ature at l.l00 l,125C. after baking in H at l.050 l.075C. for minutes.

3. Bubble H at 800 cc/minute through liquid TiCL and mix the resultant gas with 3.5 liters/minute of pure hydrogen (STP) and flow the mixture over the outer surface of thermowell for minutes at l,l()0 l,lC. and l5 inches Hg. pressure.

4. After titanizing, adjust temperature to tantalum deposition range, l,0l0 1,030C. and flow (1 at l liter/minute through heated tantalum chips to form TaCl and mix the resultant gas with 6.25 liters/minute of hydrogen passing the mixture over the thermowell surface at torr pressure. All gas flows at STP.

5. Total tantalum deposition time 75 minutes.

6. Turn off chlorine and hydrogen and cool in argon.

7. Deposit thickness from 0.007 inch at underside of hexagon adjacent to threaded coupling, to 0.010 inch 1.020 inch on remaining well surface (0.020 inch at the tip).

It will be noted that the example using stainless steel shows a longer exposure time and a lower temperature than the examples on carbon steel. This is required to avoid eutectic melting by the titanium and nickel in the steel. This melting would occur at normal" carbon steel titanizing temperatures.

I claim:

I. A composite structure having a corrosion resistant surface which comprises:

a. a non-refractory steel substrate containing free carbon and metal carbides.

b. a displacement diffusion containment coating on said substrate to isolate carbon and metal carbides from the surface of the substrate composed of titanium carbide formed from an in situ combination of titanium with the carbon and metal carbides of the substrate. said displacement diffusion coating being stable in the presence of tantalum or columbium at temperatures ranging from 900 to l,100C.. and

c. a corrosion resistant material selected from the group consisting of tantalum and columbium molecularly bonded to said containment coating by chemical vapor deposition. 

1. A COMPOSITE STRUCTURE HAVING A CORROSION RESISTANT SURFACE WHICH COMPRISES: A. A NON-REFRACTORY STEEL SUBSTRATE CONTAINING FREE CARBON AND METAL CARBIDES, B. A DISPLACEMENT DIFFUSION CONTAINMENT COATING ON SAID SUBSTRATE TO ISOLATE CARBON AND METAL CARBIDES FROM THE SURFACE OF THE SUBSTRATE COMPOSED OF TITANIUM CARBIDE FORMED FROM AN INSITU COMBINATION OF TITANIUM WITH THE CARBON AND METAL CARBIDES OF THE SUBSTRATE, SAID DISPLACEMENT DIFFUSION COATING BEING STABLE IN THE PRESENCE OF TANTALUM OR COLUMBIUM AT TEMPERATURES RANGING FROM 900* TO 1,100*C., AND C. A CORROSION RESISTANT MATERIAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM AND COLUMBIUM MOLECULARLY BONDED TO SAID CONTAINMENT COATING BY CHEMICAL VAPOR DEPOSITION. 